Acquisitions and Divestitures - Consolidated Business Acquisitions (Details) - USD ($) $ in Thousands		1 Months Ended				3 Months Ended								12 Months Ended
	Feb. 21, 2018	Mar. 31, 2019	Jan. 31, 2019	Oct. 31, 2018	Jul. 31, 2018	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017	Oct. 01, 2018	Oct. 31, 2017
Acquisitions
Compensation expense														$ 15,488	$ 7,515	$ 5,012
Sales and results of operations
Sales						$ 471,198	$ 382,679	$ 337,339	$ 305,259	$ 379,709	$ 296,212	$ 278,586	$ 248,391	1,496,475	1,202,898	1,107,709
Operating income (loss)						58,619	34,725	(6,541)	(566)	27,673	10,290	(1,679)	(11,902)	86,237	24,382	2,628
Net income (loss) after taxes						41,763	$ 23,910	(9,392)	$ (6,587)	17,816	$ 6,291	$ (2,011)	$ (9,786)	49,694	12,310	(11,209)
Loss (gain) for changes in fair values of contingent consideration														(1,005)	1,029	(3,878)
Purchase price allocation
Goodwill						578,097				$ 333,626				578,097	333,626	321,562	$ 333,626
Estimated amortization expense related to the intangible assets
2020						36,437								36,437
2021						30,454								30,454
2022						26,774								26,774
2023						19,504								19,504
2024						16,976								16,976
Thereafter						35,468								35,468
Nuvotronics
Sales and results of operations
Sales														7,400
Operating income (loss)														(6,900)
Net income (loss) after taxes														(6,900)
Amortization														1,200
Acquisition-related expenses														$ 3,000
Estimated acquisition-date fair value of consideration
Fair value of consideration transferred		$ 66,800
Cash consideration paid		61,500
Contingent consideration		4,900						4,900
Estimated Payable		400						400
Period for contingent consideration contracts														12 months
Purchase price allocation
Accounts receivable and contract assets		2,600						2,600
Fixed assets		2,700						2,700
Accounts payable and accrued expenses		(1,800)						(1,800)
Deferred taxes		(3,200)						(3,200)
Other net assets acquired (liabilities assumed)		600						600
Net identifiable assets acquired		26,400						26,400
Goodwill		40,400						40,400
Net assets acquired		$ 66,800						66,800
Weighted average useful life of intangible assets		9 years
Estimated amortization expense related to the intangible assets
2020						4,000								$ 4,000
2021						3,000								3,000
2022						3,000								3,000
2023						2,900								2,900
2024						2,700								2,700
Thereafter						10,100								10,100
Nuvotronics | Maximum
Estimated acquisition-date fair value of consideration
Contingent consideration		$ 8,000				8,000		8,000						8,000
Nuvotronics | Trade names
Purchase price allocation
Indefinite intangible assets		1,500						1,500
Nuvotronics | Technology
Purchase price allocation
Amortizable intangible assets		22,700						22,700
Nuvotronics | Customer relationships
Purchase price allocation
Amortizable intangible assets		600						600
Nuvotronics | Backlog
Purchase price allocation
Amortizable intangible assets		1,400						1,400
Nuvotronics | Non-compete agreements
Purchase price allocation
Amortizable intangible assets		$ 500						$ 500
GRIDSMART
Sales and results of operations
Sales														20,600
Operating income (loss)														900
Net income (loss) after taxes														900
Amortization														4,000
Acquisition-related expenses														2,900
Estimated acquisition-date fair value of consideration
Fair value of consideration transferred			$ 86,800
Purchase price allocation
Accounts receivable			1,700
Inventory			4,300
Accounts payable and accrued expenses			(1,900)
Deferred taxes			(3,300)
Other net assets acquired (liabilities assumed)			600
Net identifiable assets acquired			33,100
Goodwill			53,700
Net assets acquired			$ 86,800
Weighted average useful life of intangible assets			8 years
Estimated amortization expense related to the intangible assets
2020						5,300								5,300
2021						3,900								3,900
2022						3,500								3,500
2023						3,500								3,500
2024						3,500								3,500
Thereafter						8,100								8,100
GRIDSMART | Trade names
Purchase price allocation
Indefinite intangible assets			$ 2,400
GRIDSMART | Technology
Purchase price allocation
Amortizable intangible assets			25,700
GRIDSMART | Customer relationships
Purchase price allocation
Amortizable intangible assets			$ 3,600
Trafficware
Sales and results of operations
Sales														53,800
Operating income (loss)														(11,000)
Net income (loss) after taxes														(11,000)
Amortization														15,300
Acquisition-related expenses														5,200
Estimated acquisition-date fair value of consideration
Fair value of consideration transferred				$ 237,200
Purchase price allocation
Accounts receivable				10,400
Inventory				9,900
Accounts payable and accrued expenses				(8,900)
Other net assets acquired (liabilities assumed)				2,000
Net identifiable assets acquired				84,000
Goodwill				153,200
Net assets acquired				$ 237,200
Weighted average useful life of intangible assets				7 years
Estimated amortization expense related to the intangible assets
2020						11,400								11,400
2021						11,400								11,400
2022						11,400								11,400
2023						6,400								6,400
2024						5,900								5,900
Thereafter						12,900								12,900
Trafficware | Trade names
Purchase price allocation
Indefinite intangible assets				$ 4,600
Trafficware | Technology
Purchase price allocation
Amortizable intangible assets				43,300
Trafficware | Customer relationships
Purchase price allocation
Amortizable intangible assets				21,900
Trafficware | Backlog
Purchase price allocation
Amortizable intangible assets				$ 4,800
Shield Aviation
Sales and results of operations
Operating income (loss)														(5,300)	(800)
Net income (loss) after taxes														(5,300)	(600)
Amortization														800	100
Loss (gain) for changes in fair values of contingent consideration														(1,800)	200
Estimated acquisition-date fair value of consideration
Fair value of consideration transferred					$ 12,800
Extinguishment of debt					5,200
Cash consideration paid					1,300
Hold back consideration					700
Contingent consideration					5,600
Purchase price allocation
Other net assets acquired					300
Net identifiable assets acquired					6,300
Goodwill					6,500
Net assets acquired					12,800
Estimated amortization expense related to the intangible assets
2020						800								800
2021						800								800
2022						800								800
2023						800								800
2024						800								800
Thereafter						1,400								1,400
Shield Aviation | Maximum
Estimated acquisition-date fair value of consideration
Contingent consideration					10,000	10,000								10,000
Shield Aviation | Technology
Purchase price allocation
Amortizable intangible assets					$ 6,000
Weighted average useful life of intangible assets					8 years
MotionDSP
Sales and results of operations
Sales														1,500	600
Operating income (loss)														(600)	(2,700)
Net income (loss) after taxes														(600)	(1,900)
Amortization														700	400
Acquisition-related expenses														400	$ 800	$ 200
Estimated acquisition-date fair value of consideration
Cash consideration paid	$ 4,800
Purchase price allocation
Accounts payable and accrued expenses	(300)
Other noncurrent liabilities	(800)
Other net assets acquired (liabilities assumed)	900
Net identifiable assets acquired	2,800
Goodwill	6,700
Net assets acquired	$ 9,500
Weighted average useful life of intangible assets	7 years
Estimated amortization expense related to the intangible assets
2020						700								700
2021						700								700
2022						700								700
2023						700								700
2024						600								600
Thereafter						$ 200								$ 200
MotionDSP | Trade names
Purchase price allocation
Indefinite intangible assets	$ 100
MotionDSP | Technology
Purchase price allocation
Amortizable intangible assets	4,500
MotionDSP | Customer relationships
Purchase price allocation
Amortizable intangible assets	$ 200
MotionDSP
Acquisitions
Purchase price of capital stock																		$ 4,700
Ownership percentage																		49.00%